The system is pushed by a force $\mathrm{F}$ as shown in the figure. All surfaces are smooth except between $\mathrm{B}$ and $\mathrm{C}$. Friction coefficient between $\mathrm{B}$ and $\mathrm{C}$ is $\mathrm{\mu }$. Minimum value of $\mathrm{F}$ to prevent block $\mathrm{B}$ from downward slipping is